Metering expansion nozzle for CO2

ABSTRACT

An expansion nozzle (20) for making carbon-dioxide snow from liquid carbon dioxide, for spraying refrigerated or frozen items (2), nozzle has an integrated metering valve (30) with a discharge aperture (32), an intake-side valve seat (33), and a valve needle (31) adjustable relative to the valve seat (33). The needle projects into the discharge aperture (32). A jet shaper (60) adjoins the discharge channel (35), in which the liquid carbon dioxide expands within a cross section widened at an end thereof. An expansion region (62) with a diverging cross section has a rectangular flow cross section.

The invention relates to an expansion nozzle and a method for makingcarbon-dioxide snow for spraying refrigerated or frozen items.

It is known from U.S. Pat. No. 3,815,377 to cool or shock-freezerefrigerated or frozen items, for example food items, by spraying themwith carbon-dioxide snow produced through the expansion of liquid carbondioxide in an expansion nozzle. For this purpose, the refrigerated orfrozen items are typically located in a refrigeration or frosterchamber, into which a supply line for liquid carbon dioxide leads; theexpansion nozzle is disposed at the end of this line in therefrigeration or froster chamber. A throttle valve that regulates theflow-through is usually provided in the supply line. The CO₂ is held,normally in liquid form, in a storage tank, where it is maintained inequilibrium with the CO₂ gas phase above the liquid in the tank. Thepressure in the tank is usually maintained at about 18 bar, and thetemperature is maintained at about -23° C. In the systems known up tonow, usually two pipelines lead from the tank to the refrigeration orfroster chamber. In one line, the liquid CO₂ flows to the refrigerationor froster chamber, where it makes available the necessary coolingcapacity. The second pipeline leads from the upper gas region of thetank to the refrigeration or froster chamber, and is connected to thefirst line by way of a stop valve shortly before it enters therefrigeration or froster chamber. The tank and the first line areinsulated with a thick insulating layer, for example comprising PU foam,against the penetration of heat or loss of cold. Gaseous CO₂ flows tothe refrigeration or froster chamber through the second pipeline, whichis not insulated. This serves to rinse the part of the liquid pipelinedisposed behind the metering valve, as well as the expansion elements.To attain and maintain the desired operating temperature in therefrigeration or froster chamber, CO₂ is usually sprayed into therefrigeration or froster chamber in cycles. To this end, a temperaturemeasurement is performed in the refrigeration or froster chamber. If thetemperature exceeds a set limit value, the supply of cooling medium isinitiated, that is, the valves for liquid CO₂ in the supply line areopened. If the temperature is below a set limit value, the supply ofcooling medium is cut off, that is, the valves are re-closed. Theinitiation and cutoff of cooling medium supply is typically effectedwith magnet valves disposed in the supply line, outside of therefrigeration or froster chamber. After the supply of cooling medium hasbeen cut off, the pressure in the adjoining pipe and the expansionelements drops to an ambient pressure of about 1 bar. If the pressuredrops below the triple-point pressure of 5.18 bar while liquid CO₂ isstill in the supply line, a phase conversion into solid CO₂ snow occurs.This solid CO₂ snow can no longer exit the expansion elements, therebyclogging the system. In an unfavorable case, this can lead to a blockageof the valves and, in an extreme case, large portions of the entirepipeline. The pipelines must then be closed at the tank and thawed overa period of several hours. Furthermore, in an extreme case of blockage,the danger of a line rupture exists, because pressures of up to 60 barcan be present in the pipelines. To prevent such clogs, the liquid linebehind the valve and the expansion element are re-rinsed by means of thegaseous CO₂ from the second line. The CO₂ gas flowing in forces theliquid out of the line system, and prevents a pressure drop below thetriple point as long as CO₂ liquid is in the pipeline.

A disadvantage of this apparatus is that additional cooling medium isrequired for rinsing the liquid line and maintaining the pressure in theliquid line with the CO₂ gas. This CO₂ gas does not contribute to thecooling capacity of the froster. To the contrary, the CO₂ gas used inthe re-rinsing process additionally warms the atmosphere in therefrigeration or froster chamber, so the pause cycles, during which nocooling medium is sprayed in, are shortened, and the cooling-mediumconsumption is undesirably high. A further disadvantage is that thetemperature in the refrigeration or froster chamber is not constant overtime, but fluctuates between an upper and a lower set limit value. As aresult, a temperature fluctuation likewise occurs in the refrigerationor froster chamber, which affects product quality. Such temperaturefluctuations can be reduced, however, by the setting of the upper andlower limit values in a small range; however, this again reduces theperiod of a cooling cycle and thus the switching frequency for thespraying cycles. Because re-rinsing with CO₂ gas must be performed aftereach spraying cycle, the losses due to the warm CO₂ gas increase. Up tothis point, a reliable, continuous metering without clogging of theexpansion elements was not possible in the apparatuses and expansionnozzles with the cooling medium CO₂ occurring in three phases.

It is the object of the invention to create an expansion nozzle and amethod of the type mentioned at the outset, so that, with an inexpensiveand simple design of the expansion nozzle or the entire coolingapparatus, the cooling-medium consumption is reduced and temperaturefluctuations in the refrigeration or froster chamber are virtuallyavoided completely, assuring uniform product quality.

This object is accomplished with respect to the apparatus in that theexpansion nozzle includes an integrated metering valve that has adischarge aperture, a valve seat on the intake side, and a valve needlethat is adjustable relative to this valve seat and projects into thedischarge aperture of the metering valve in the closed state, closingthe metering valve in the discharge direction.

With respect to the method, the object is accomplished by thecontinuously-variable metering of the carbon-dioxide mass flow directlyin the expansion nozzle.

With the expansion nozzle of the invention, it is possible to achieve acontinuous supply of cooling medium with constant regulation, inaccordance with the method of the invention, as opposed tointermittently supplying the carbon dioxide. This regulation is effecteddirectly at the outlet of the CO₂ line. Thus, re-rinsing with CO₂ gas isno longer necessary. This not only reduces the cooling-mediumconsumption and increases the froster capacity, but simultaneouslyeliminates the gas line between the supply tank and the refrigeration orfroster chamber from the design of the apparatus. With the constantcooling-medium metering, the temperature distribution is more precise,more uniform and constant over time in the refrigeration or frosterchamber, and therefore in the refrigerated or frozen items. Thiscontributes to increased product quality. The sprayed quantity can becompletely arbitrarily constant or variable. The jet is advantageouslysteady, and not pulsating. No additional device is necessary forattaining the pressure in front of the expansion nozzle to prevent thepressure from falling below the triple point.

The dependent claims describe advantageous embodiments and modificationsof the expansion nozzle and method of the invention.

The valve seat has a discharge funnel that narrows conically in thedischarge direction, and an adjoining, short discharge channel. Themetering valve preferably has a valve base with a valve chamber that isopen to one side, and a cooling-medium connection aperture that leads tothe valve chamber. The cooling-medium connection aperture preferablyterminates into the valve chamber laterally and at an incline to thedirection of the discharge aperture. The inclined position of thecooling-medium connection ensures that the liquid carbon dioxide ispartially adapted to the flow direction in the valve base and theadjoining valve seat.

The valve needle is seated to be displaceable in the longitudinaldirection of the needle relative to the open side of the valve chamber,and the tip of the needle projects out of the open side of the valvechamber, at least in the closed position of the valve needle. In theassembled state of the metering valve, the open side of the valvechamber is covered by a valve-seat plate that contains the valve seat,and is preferably detachably connected to the valve base with the use ofa seal. This design permits a simple exchange of parts that are subjectto wear, such as the valve needle and valve seat. For assembly, when thevalve needle is in place, the valve-seat plate is simply pushed over theneedle tip and is thereby automatically centered tightly against theneedle. Care must be taken to ensure that the needle can move axiallyfreely during assembly to prevent the needle from bending duringmounting.

Opposite the valve seat, the valve needle is guided out of the valvebase in a guide channel that extends outward from the valve chamber. Thevalve-needle head projecting out of the valve base is coupled to anactuating drive, for example a stepped motor. The actuating drive isused to move the valve needle axially. Because of the axial movement ofthe valve needle in the valve seat, the flow cross section can be variedvery precisely. In this way, it is possible to regulate thecooling-medium mass flow. At the same time, the valve needle assumes thetask of sealing the cooling-medium supply line in connection with thevalve seat, in that the actuating drive presses the valve needle intothe nozzle seat, creating an annular seal.

Without a further, downstream component, the valve generates acircular-ring-shaped cooling-medium jet comprising CO₂ gas and CO₂ snow.The shape of the jet changes with increasing distance from the meteringvalve. The free core of the circular jet disappears with increasingdistance, so a circular jet forms from the circular ring. The flow crosssection corresponds to the flow cross section of a conventional nozzlehaving a cylindrical flow cross section. In an arrangement of suchexpansion nozzles above a conveyor belt with a jet orientation of about45° with respect to the vertical, elliptical spray surfaces are createdfor the refrigerated or frozen items on the conveyor belt. The time inwhich a certain refrigerated or frozen item is located on the conveyorbelt in the cold spray jet is therefore dependent on the position on theconveyor belt, because the path that the refrigerated or frozen itemsmust traverse through the jet also changes. If, for example, therefrigerated or frozen items are conveyed through the center of theelliptical spray jet, maximum cooling is effected, because the paththrough the spray jet is of maximum length. At the edge of the sprayjet, in contrast, the refrigerated or frozen items are only cooledslightly, because the cooling phase does not last as long there due tothe shorter path.

For this reason, it is particularly advantageous for a nozzle sectionwhose cross section widens at the end side to adjoin the dischargechannel as a jet shaper, in which the liquid carbon dioxide is expandedto form the jet. This is an inventive concept in and of itself.

On the side of the valve, the jet shaper preferably has a short,cylindrical channel that forms a nozzle throat, and an adjoiningexpansion region having a diverging flow cross section whose dimensionsare selected corresponding to the desired jet shape. A rectangular flowcross section is particularly preferable here. This cross section isadvantageously created by an expansion region, for example, that isembodied as a slot having two opposite, diverging sides and twoopposite, parallel sides.

The diameter of the nozzle throat is selected as a function of thenominal width of the metering valve and the desired operating parametersin the refrigeration or froster chamber, and the length of the nozzlethroat is selected as a function of the nominal width of the meteringvalve, the angle of the valve needle and the stroke of the actuatingdrive of the valve needle, such that the pressure of the carbon dioxideflowing through the nozzle throat does not drop below the triple-pointpressure of the carbon dioxide within the nozzle throat. The nozzlethroat of the jet shaper and the discharge channel of the valve seatpreferably have the same diameter. In the assembled state of theexpansion nozzle, the nozzle throat seamlessly adjoins the dischargechannel. The total length of the nozzle throat and the discharge channeltogether should preferably be dimensioned according to the formula##EQU1## Here D is the diameter of the nozzle throat and the dischargechannel, respectively, and α is the angle of the needle tip. A deviationof approximately 10% in either direction from this value is acceptable,however. Only gaseous and liquid carbon dioxide flow through the nozzlethroat, so the cooling medium can be further metered continuouslywithout the occurrence of CO₂ snow clogs or deposits in the nozzlethroat or discharge channel of the valve seat. The pressure first dropsbelow the triple point inside the diverging expansion regions, and solidCO₂ snow particles form with a sudden increased formation of CO₂ gas.

As an alternative, it is also possible in principle to embody the nozzlethroat and/or the discharge channel such that the total length canchange as the position of the valve needle changes. In this instance,the length of the flow channel should decrease corresponding to theaxial needle stroke when the valve needle is open.

Because the expansion region is designed to diverge, the CO₂ snowparticles are directed away from the nozzle slot; thus, no or onlysecondary velocity components occur in the direction of the slot walls.This prevents clogging. With the widening of the jet, the velocitylikewise increases in the axial direction, and transversely thereto. TheCO₂ snow particles and the cold CO₂ gas therefore receive a strongerpulse, so the jet retains its direction and expansion after exiting theslot. The embodiment of the expansion region as a slot advantageouslypermits the formation of a flat jet having a rectangular spray crosssection. In principle, the jet can be made wide enough that it extendsover the entire width of the conveyor belt.

Hence, with the special arrangement of the metering valve and theadjoining jet shaper of the expansion nozzle according to the invention,in the expansion nozzle, the carbon-dioxide jet is first guided inannular fashion around the needle tip and into the discharge channel ornozzle throat, then guided together there and subsequently expandedpurposefully, forming the carbon-dioxide snow. This prevents thepressure from falling below the triple point within the dischargechannel or nozzle throat.

The metering valve and the jet shaper are advantageously flanged onebehind the other as modular units of the expansion nozzle, with theconnecting surfaces of the metering valve and the jet shaper preferablybeing nested and centered with a fit. It is therefore possible toselectively equip the metering valve with the corresponding, desired jetshaper, for example with the appropriate width.

The invention, of course, also encompasses a corresponding apparatus forcooling or shock-frosting refrigerated or frozen items, having at leastone refrigeration or froster chamber, at least one supply line forliquid carbon dioxide that leads into the refrigeration or frosterchamber, at least one expansion nozzle that is disposed at the end ofthe supply line in the chamber, and a metering valve that regulates theflow-through in the supply line, with the expansion nozzle containingthe metering valve as an integrated component and being embodiedaccording to the above-described invention.

An independent inventive concept within this apparatus is that a gasseparator is interposed in the supply line for the liquid carbondioxide; the separator separates the gas components in the liquid carbondioxide. Due to the pressure drop and the heat flow into the pipeline, aformation of gas in the bubbling cooling medium removed from the tank isunavoidable without an external pressure increase or cooling of thepipeline. First, small gas bubbles form in the cooling medium, whichgrow very rapidly as they flow further through the pipe due to theconsiderably larger specific volume, and combine to form larger bubbles.Thus, entire regions of gas form from piston and plug bubbles; evenlengthy segments of the pipe are partially filled with gas. The ratio offormed gas to the total mass flow is particularly high in thincooling-medium supply lines. Because of the greatly-differing physicalcharacteristics of gas and liquid, different flow ratios occur in thepipeline and the adjoining expansion nozzle. For example, the flowvelocity and the associated volume flow in the gas flow-through (up to220 m/sec) are significantly higher than in the liquid flow-through(roughly between 15 m/sec and 75 m/sec). The occurring fluctuations invelocity additionally cause an unsteady pressure loss in thecooling-medium supply line, which further intensifies the effect oflarge gas bubble formation. The pulsation-type expulsion of thecooling-medium jet from the expansion element also leads to an increasednoise emission in comparison to the flow of single-phase cooling media.An increased noise level causes unpleasant stress for the operatingpersonnel. Additionally, interferences and sources of danger cannot beperceived as readily. The danger associated with the increased flowvelocity due to the bubbles is that sensitive, soft refrigerated orfrozen items will be damaged. Lightweight refrigerated or frozen itemscan be spun by a powerful cooling-medium jet of the froster, andlikewise be damaged or destroyed.

Through the separation of the gas phase from the liquid, a single-phaseflow is achieved, which avoids the problems of inhomogeneous flows. Theaverage flow velocity is reduced, so the pressure difference at thevalve seat and in the nozzle throat is less than in an inhomogeneousflow. Thus, a higher absolute pressure is present at the end of thenozzle throat. For this reason, the pressure in the supply tank can beset lower than that of the liquid-gas mixture flow, that is, less than18 bar, without the pressure being below the triple-point pressure inthe nozzle throat. With the reduction in the tank pressure, theproportion of solid CO₂ snow that forms during the expansion increasesagain, which is in turn associated with a higher cooling capacity withthe same quantity of cooling medium. The gaseous carbon dioxide that wasseparated in the gas separator can preferably be conducted into therefrigeration or froster chamber by way of a separate gas line forattaining the total cooling capacity of the supplied cooling medium. Afurther advantage of this gas separator is that the noise emissions arereduced.

The invention is described in detail below by way of embodiments, withreference to the attached drawings. Shown are in:

FIG. 1 a schematic representation of the apparatus of the invention,

FIG. 2 a section through a metering valve of the invention,

FIG. 3a a view from below of a rectangular, slot-like jet shaper,

FIG. 3b a section through the jet shaper of FIG. 3a, transversely to thelongitudinal direction of the slot along arrows IIIb--IIIb,

FIG. 3c a section through a jet shaper according to FIG. 3a, in thelongitudinal direction of the slot along arrows IIIc--IIIc,

FIG. 4 a schematic representation of the gas separator of the invention,

FIG. 5 a schematic plan view of a conveyor belt on which refrigerated orfrozen items are disposed, in the refrigeration or froster chamber, andthe jet geometry of the carbon-dioxide jet sprayed onto the conveyorbelt.

FIG. 1 shows an apparatus (1) for shock-frosting refrigerated or frozenitems (2), having a refrigeration or froster chamber (3), a supply line(4) for liquid carbon dioxide (CO₂), which leads into the refrigerationor froster chamber (3), and an expansion nozzle (20), which is disposedat the end of the supply line in the refrigeration or froster chamber(3), and in which the liquid carbon dioxide is expanded. The resultingjet of gaseous CO₂ and CO₂ snow is sprayed onto a conveyor belt (5)located in the refrigeration or froster chamber (3), on which therefrigerated or frozen items (2) are conveyed away under the expansionnozzle (20).

The liquid CO₂ is first held in a storage tank (80). Here it is inequilibrium with the CO₂ gas phase above the liquid in the tank (80). Acontrolled cooling aggregate (85), which preferably maintains thepressure in the tank (80) in a range of about 18 bar, is disposed at thetank (80). A temperature of about -23° C. is present. Further disposedat the tank (80) is a weighing device (85), which can be used to checkthe tank's contents, so that the cooling medium can be allocated timelywhen needed. The tank (80) itself is thermally insulated againstpenetrating heat, and therefore cold losses, by an approximately 160-200mm thick insulating layer of PU foam.

An insulated pipeline (4) for the liquid CO₂ leads from the tank (80) tothe expansion nozzle (20) in the refrigeration or froster chamber (3). Ablocking element (82) is disposed in the liquid pipeline (4), directlybehind the tank (80). A blocking element (83) and an overpressure valve(87) are likewise disposed directly behind the tank (80) in the gas line(84). Before the apparatus is started up, when the blocking element (82)in the is closed, the liquid pipeline (4) is pre-stressed with gaseousCO₂ by the opening of the blocking element (83) in the gas line (84),that is, the carbon-dioxide supply line (4) is brought under the samepressure as the tank (80). Subsequently, the stop valve (83) in the gasline (84) must be re-closed, and the liquid stop valve (82) can beopened. Without this pre-stressing with CO₂ gas, such a severe pressuredrop would occur upon the opening of the liquid stop valve (82) that thepressure would still be below the triple-point pressure of the CO₂ of5.18 bar within the supply line (4), and CO₂ snow would form in the line(4) and clog the pipeline (4).

A gas separator (70) is interposed in the cooling-medium supply line(4), in front of the refrigeration or froster chamber (3). The gascomponents contained in the liquid carbon dioxide are separated in theseparator.

The gas separator (70) comprises a cylindrical pre-separation chamber(71) having intake connections (73) for the two-phase carbon dioxide,the connections being disposed laterally in the upper region, and adischarge nozzle (74) for the degassed liquid carbon dioxide in thelower region. The intake connection (73) is disposed at an acute angleto the pre-separation chamber (71), preferably less than 10°, anddiagonally upward and tangentially to the longitudinal axis of thepre-separation chamber (FIG. 4). A circulation flow is effected by theinflux of the CO₂ tangentially and from below. The occurring centrifugalforces cause the liquid to contact the walls of the pre-separationchamber (71), and gas to collect in the center, from where it isconducted away by a control head (72) disposed above the pre-separationchamber (71) and having a regulated discharge valve. The pre-separationchamber (71) is partitioned into two partial chambers (75, 76) by ahorizontal partition wall (77), which is disposed below the intakeconnection (73) and provided with apertures. The partition wall is aperforated or slotted sheet. It is also possible to use a plurality ofpartition walls to divide the chamber further into a plurality ofsuperposed chambers. This partitioning causes the gas to collect in theupper part of the pre-separation chamber (71).

The control head (72) comprises a float chamber having a discharge valveat the top, whose closing element is coupled to a float located in thefloat chamber. If gas collects in the control head (72), the float sinksand opens the valve, so the gas can be released through the control head(72). The separated gas is supplied to the refrigeration or frosterchamber (3) through a separate supply line (78) that has a magnet valve(79) at one end. This line (78) is preferably also insulated againstheat penetration. In a particularly preferred embodiment, the liquid andgaseous carbon dioxide exiting the gas separator (70) are conducted tothe refrigeration or froster chamber (3) in a double pipeline formed bytwo coaxially-nested pipes, with the gaseous carbon dioxide beingconducted through the outside, annular pipe. This minimizes cold losses.

A magnet valve (not shown) can likewise be disposed in thecooling-medium supply line (4), in front of the refrigeration or frosterchamber (3), for additionally closing the supply line (4) during alengthy pause in operation.

The expansion nozzle (20) of the invention comprises a metering valve(30) that is disposed at the end side at the supply line (4), and adownstream jet shaper (60), which are flanged one behind the other asmodular units (30, 60).

The metering valve (30) comprises a valve base (40) having anessentially cylindrical valve chamber (41) that is open to one side, anda cooling-medium connection aperture (42) that leads to the valvechamber (41). The open side of the valve chamber (41) of the meteringvalve (30) is covered by a valve-seat plate (36), which is detachablyconnected to the valve base (40) with the use of a seal (39). The valveseat (33), which has a discharge funnel (34) that narrows conically inthe discharge direction, and an adjoining, short discharge channel (35),is disposed in the valve-seat plate (36). The discharge channel (35) iscoaxial to the longitudinal axis of the valve chamber (41). The crosssection of the discharge funnel (34) on the side of the valve chambercorresponds to the cross section of the valve chamber (41) in the valvebase (40). A valve needle (31) that is seated to be displaceable in thelongitudinal direction of the needle extends coaxially through the valvechamber (41) in the valve base. In the closed position, the tip of theneedle projects out of the open side of the valve chamber (41) and intothe discharge aperture (32) of the metering valve (30). Opposite thevalve seat (33), the valve needle (31) is guided out of the valve base(40) in a guide conduit (44) extending from the valve chamber (41). Thevalve-needle head (31K) projecting out of the valve base (40) is coupledto an actuating drive used to move the valve needle (31) axially in themetering valve (30) for regulating the flow of cooling-medium mass, andto press the needle into the valve seat (33) for closing thecooling-medium line (4).

The guide channel (44) is separated from the valve chamber (41) by aconstriction formed by an annular, radially inward-extending web (45). Afirst guide bushing (46), a spring element (49), a counter-ring (50), aplurality of annular seals (roof collars) (48) having a roof-shapedcross section, and a second guide bushing (47) with a bracket ring (51)that rests against the upper roof collar (48) are inserted in thissequence into the guide channel (44) over the valve needle (31) from thevalve-needle head (31K). The valve needle (31) is held axially movablyand with virtually no radial play in the guide bushings (46, 47), whichare preferably produced from PTFE. A pressure bushing (52) is pushedover the valve needle (31) above the second guide bushing (47). A unionnut (53), which is screwed on above the valve base (40) from above,presses the pressure bushing (52), the second guide bushing (47) withthe bracket ring (51), the roof collars (48) and the counter-ring (50),counter to the spring force, against the first guide bushing (46) seatedon the annular web (45). The compression of the roof collars (48) causesthe inside edge of the roof collars (48) to be pressed tightly againstthe valve needle (31) and the outer edge of the roof collars (48) to bepressed tightly against the inside wall of the guide channel (44). Inthis way, the valve chamber (41) is sealed toward the outside against anominal pressure of up to 20 bar. At the same time, the valve needle(31) is additionally guided by the roof collars (48). of course,commercially-available glands, for example, can also be used for sealinginstead of the roof collars (48).

The valve-seat plate (36) is screwed directly under the valve base (40)by three screws. The system is designed such that a radial play of thevalve-seat plate (36) exists when the fastening screws loosen slightly.This allows the valve-seat plate (36) to be centered with respect to thevalve needle (31). In the assembly of the valve (30), first the valveneedle (31) is inserted into the valve base (40) and moved outward inthe axial direction, to the position it assumes later, when the valve(30) is closed. The valve-seat plate (36) can then be pressed againstthe valve needle (31), under the valve base (40), and is automaticallycentered. When the fastening screws of the valve-seat plate (36) aretightened, the valve needle (31) should be axially movable to preventthe valve needle (31) from bending. A seal (39) is disposed between thevalve-seat plate (36) and the valve base (40).

The embodiment of the valve seat (33) as a discharge funnel (34) thatnarrows in the discharge direction and has an adjoining, short dischargechannel (35) ensures that a uniformly-accelerated flow is achieved inthe valve. An opening angle of the discharge funnel (34) between about56° and about 76°, preferably 66°, has proven particularly advantageous.The valve needle (31) should preferably converge to a point at an anglebetween about 16° and about 30°. The valve seat (33) is advantageouslymanufactured from a material whose elastic properties permit a tightclosure of the valve needle (31) and, despite its soft nature, exhibitsno increased wear due to abrasive solid particles. As in the selectionof the other materials, considerations include the durability of thefood items and resistance to corrosion. Polycarbonate has provenparticularly well-suited for use as a valve-seat material.

The cooling-medium connection aperture (42) is guided, at an incline,laterally into the valve chamber (41) in the direction of the dischargeaperture (32). Thus, the cooling medium flowing in is partially adaptedto the flow direction in the valve base (40).

When the expansion nozzle (20) is assembled, the discharge channel (35)of the valve seat (33) makes a seamless transition into a short,cylindrical nozzle throat (61) of the jet shaper (60) disposed behindit. Adjoining the nozzle throat (61) is an expansion region (62), whichhas a diverging flow cross section, and in which the liquid carbondioxide is expanded to form the jet; the dimensions of the expansionregion are selected corresponding to the desired jet shape. Care must betaken to ensure that the tip of the valve needle (31) does not projectinto the expansion region (62), if possible, during assembly.

According to FIGS. 3a through 3c, the expansion region (62) has arectangular flow cross section, and is embodied as a slot (62) havingtwo opposite, diverging sides (64) and two opposite, parallel sides(65).

The length and diameter of the nozzle throat (61) and the dischargechannel (35) of the valve seat (33), respectively, are selected suchthat the pressure of the carbon dioxide flowing through the nozzlethroat (61) does not drop below the triple-point pressure of the CO₂within the nozzle throat (61), so no CO₂ snow is formed here. The totallength L of the nozzle throat (61) and the discharge channel (35),respectively, is to be selected according to the formula ##EQU2## with adeviation of ±10%, where D is the diameter of the nozzle throat (61) andthe discharge channel (35), respectively, and α is the angle of thevalve-needle tip.

The CO₂ flows from the nozzle throat (61) into the slot (62), where itfills the entire cross section of the slot. Inside the nozzle slot, thepressure then drops below the triple point, causing a phase conversionof liquid CO₂ into solid CO₂ snow particles with a sudden increasedformation of CO₂ gas. Because the nozzle slot (62) diverges, no clogsform inside the nozzle slot (62). Moreover, with the widening of thejet, the velocity increases in the axial direction and transverselythereto, so the CO₂ snow particles and the cold CO₂ gas receive a largerpulse. This causes the jet to retain its direction and expansion afterexiting the slot (62). It is therefore possible to produce a flat jethaving a rectangular spray cross section that extends over the entirewidth of the conveyor belt (5) in the refrigeration or froster chamber(3).

The cooling capacity of a cooling-medium jet depends on the quantity ofsprayed CO₂ gas and the loading with CO₂ snow particles. This appliesfor both the entire jet and each individual angle of the jet. If thedistribution of gas and snow is not uniform over the width of the jet,regions of varying cooling intensity form on the conveyor belt (5), andthus also in the refrigerated or frozen items (2). For this reason, theside walls (64, 65) of the jet shaper (60) are preferably straight. Itis, however, also possible in principle to embody the diverging sidewalls (64) to be concave, or provide them with a plurality of adjacent,concave niches (not shown). The transition from the nozzle throat (61)into the slot (62) is sharp-edged for avoiding an increased snowconcentration at certain jet angles. The outlet of the jet shaper (60)is likewise sharp-edged. This causes the exiting CO₂ jet to break awayat the edge (67). The jet retains its direction and shape because of thereceived pulse. The edges of the jet can therefore be set veryprecisely. To prevent the jet shaper (60) from clogging due to thefriction of the liquid and the CO₂ particles at the wall of the slot(62), the slot width should advantageously be larger than the diameterof the valve seat (33).

The total length of the slot (62) in the jet direction should beselected as a function of the pressure in front of the nozzle (20) andthe total mass flow.

The inside surface of the slot (62) is advantageously produced with theleast possible surface roughness to impede sticking of CO₂ snowparticles to the surface.

With the jet shaper (60), it is possible to give the three-phasecooling-medium jet a desired shape. This is not attained through thediversion of a predetermined jet, but through the self-shaping of thecooling-medium jet as a consequence of the purposeful re-expansion.

To create a uniform jet, counter to the actual goal, the jet is firstguided together as an annular jet emerging from the metering valve (30)so that it can be distributed uniformly from the flow core.

The connection surface (37) of the metering valve (30) is provided witha fit or annulus or rim (38), so the jet shaper (60) can likewise beconnected, centered, to the metering valve (30) by three screws. Thesethree screws for connecting the metering valve to the jet shaper (60)are correspondingly offset from the three screws used to secure thevalve-seat plate (36) to the valve base (40). A seal (63) can also bedisposed between the jet shaper (60) and the metering valve (30).

In a preferred embodiment according to FIG. 5, two expansion nozzles(20) are disposed one behind the other in the transport direction (R) inthe refrigeration or froster chamber (3), above the conveyor belt (5).One of the expansion nozzles (20) is directed at the conveyor belt (5)diagonally, at a downward 45° angle in the transport direction (R), andthe other expansion nozzle (20) is directed diagonally downward at theconveyor belt (5), counter to the transport direction (R). Thus, thefront and rear sides of the refrigerated or frozen items (2) are actedupon by the spray jet. The two expansion nozzles (20) produce arectangular cross section that extends over nearly the entire width ofthe conveyor belt (5). The refrigerated or frozen items (2) located nextto one another on the conveyor belt (5) must then always traverse thesame path through the spray jet, regardless of their position on theconveyor belt (5), effecting uniform cooling of the items (2). Ofcourse, it is also possible to combine a plurality of adjacent expansionnozzles (20) disposed transversely to the transport direction (R) toform spraying blocks for spraying wider conveyor belts.

Likewise, it is possible, of course, to embody a refrigeration orfroster chamber (3) to have only one expansion nozzle.

Ventilating fans (6) for ensuring strong swirling and thus good mixingof the froster-chamber atmosphere are disposed in the refrigeration orfroster chamber (3). This arrangement effects a uniform backgroundtemperature. Also disposed in the refrigeration or froster chamber (3)is a temperature sensor (7), which is connected to a control device (8)that regulates the actuation drive of the expansion nozzles (20). Tocarry off the incoming CO₂ again after the refrigeration or frosterchamber (3) has been heated, the refrigeration or froster chamber (3)has an outlet with a ventilating fan (9).

What is claimed is:
 1. An expansion nozzle (20) for makingcarbon-dioxide snow from liquid carbon dioxide for spraying refrigeratedor frozen items (2), wherein the nozzle has a discharge direction andcomprises:an integrated metering valve (30) including a dischargeaperture (32), an intake-side valve seat (33) and a valve needle (31)adjustable relative to the valve seat (33), the needle being adapted toclose the metering valve (30) in the discharge direction; the needleprojecting into the discharge aperture (32) of the metering valve (30)in a closed state of the metering valve; the valve seat (33) including ashort discharge channel (35); a jet shaper (60) adjoining the dischargechannel (35), in which the liquid carbon dioxide expands in a jet shape,the let shaper including a nozzle segment having a cross section widenedat an end thereof; the jet shaper (60) including on a valve side thereofa short, cylindrical channel (61) acting as a nozzle throat (61), andadjoining thereto an expansion region (62) including a diverging crosssection having dimensions being a function of the desired let shape; andwherein the expansion region (62) comprises a rectangular flow crosssection.
 2. The expansion nozzle according to claim 1, characterized inthat the valve seat (33) has a discharge funnel (34) that narrowsconically in the discharge direction, and the discharge channel (35)adjoins the funnel.
 3. The expansion nozzle according to claim 2,characterized in that the cross section of the valve chamber (41) isperpendicular to the longitudinal direction of the needle, the crosssection corresponding to the cross section of the discharge funnel (34)of the valve seat (33) on the side of the valve chamber.
 4. Theexpansion nozzle according to claim 2, characterized in that thedischarge funnel (34) of the valve seat (33) has an opening anglebetween about 56° and about 76°, preferably 66°.
 5. The expansion nozzleaccording to claim 1, characterized in that the metering valve (30)includes a valve base (40) with a valve chamber (41) that is open to oneside, and a cooling-medium connection aperture (42) that leads to thevalve chamber (41), and the valve needle (31) is seated to bedisplaceable in the longitudinal direction of the needle relative to theopen side of the valve chamber (41) in the valve base (40, and the tipof the needle projects out of the open side (43) of the valve chamber(41), at least when the valve needle (31) is in the closed position,and, in the assembled state of the metering valve (30), the open side(43) of the valve chamber (41) is covered by a valve-seat plate (36)that contains the valve seat (33) and is detachably connected to thevalve base (40).
 6. The expansion nozzle according to claim 5,characterized in that the valve needle (31) is guided out of the valvebase (40) in a guide channel (44) that extends from the valve chamber(41) opposite the valve seat (33).
 7. The expansion nozzle according toclaim 6, characterized in that the valve needle (31) is seated to beaxially movable by way of at least two guide bushings (46, 47) disposedin the guide channel (44), and at least one annular seal (roof collar)(48) that has a roof-shaped cross section is preferably disposed betweenthe guide bushings (46, 47), and is compressed axially for sealingpurposes, with the inside edge of the roof collar (48) being pressedtightly against the valve needle (31) and the outside edge of the roofcollar (48) being pressed tightly against the inside wall of the guidechannel (44).
 8. The expansion nozzle according to claim 7,characterized in that the guide channel (44) is separated from the valvechamber (41) by a constriction formed by an annular, radiallyinward-extending web (45), and a first guide bushing (46), a springelement (49), a counter-ring (50) that forms a base for the roof collar(48), at least one roof collar (48) and a second guide bushing (47) witha bracket ring (51) that rests against the roof collar (48) are insertedin this sequence into the guide channel (44) over the valve needle (31)from the valve-needle head (31K), and the second guide bushing (47), thebracket ring (51), the roof collar (48) and the counter-ring (50) arepressed, counter to the spring force, against the first guide bushing(46) seated on the annular web (45) by means of at least one screwelement and/or a clamping element (52, 53).
 9. The expansion nozzleaccording to claim 6, characterized in that the valve-needle head (31K)projecting out of the valve base (40) is coupled to an actuating drive.10. The expansion nozzle according to claim 5, characterized in that thecooling-medium connection aperture (42) terminates into the valvechamber (41) laterally and at an incline to the direction of thedischarge aperture (32).
 11. The expansion nozzle according to claim 1,characterized in that the valve needle (31) converges to a point at anangle between about 16° and about 30°.
 12. The expansion nozzleaccording to claim 1, characterized in that the expansion region (62) isembodied as a slot (62) having two opposite, diverging sides (64) andtwo opposite, parallel sides (65).
 13. The expansion nozzle according toclaim 1, to 15, characterized in that the diameter of the nozzle throat(61) is selected as a function of the nominal width of the meteringvalve (30) and the desired operating parameters in the refrigeration orfroster chamber (3), and the length of the nozzle throat (61) isselected as a function of the nominal width of the metering valve (30),the angle of the valve needle (31) and the stroke of the actuating driveof the valve needle (31) such that the pressure of the carbon dioxideflowing through the nozzle throat (61) does not drop below thetriple-point pressure of the carbon dioxide inside the nozzle throat(61).
 14. The expansion nozzle according to claim 1, characterized inthat the nozzle throat (61) of the jet shaper (60) has the same diameteras the discharge channel (35) of the valve seat (33), and seamlesslyadjoins the discharge channel in the assembled state, and the totallength of the nozzle throat (61) and the discharge channel (35)approximately corresponds to half of the diameter divided by the tangentof half of the angle of the needle tip.
 15. The expansion nozzleaccording to claim 1, characterized in that the length of the nozzlethroat (61) and/or the discharge channel (35) can be changed inassociation with the position of the valve needle (31).
 16. Theexpansion nozzle according to claim 1, characterized in that themetering valve (30) and the jet shaper (60) are flanged one behind theother as modular units (30, 60) of the expansion nozzle (20).
 17. Theexpansion nozzle according to claim 16, characterized in that theconnection surfaces (37, 66) of the metering valve (30) and the jetshaper (60) are nested, and centered, with a rim (38).
 18. An expansionnozzle (20) for making carbon-dioxide snow from liquid carbon dioxidefor spraying refrigerated or frozen items (2), wherein the nozzle has adischarge direction and comprises:an integrated metering valve (30)including a discharge aperture (32), an intake-side valve seat (33) anda valve needle (31) adjustable relative to the valve seat (33), theneedle being adapted to close the metering valve (30) in the dischargedirection the needle projecting into the discharge aperture (32) of themetering valve (30) in a closed state of the metering valve; includingat least one refrigeration or froster chamber (3), at least one supplyline (4) for liquid carbon dioxide (CO₂) that leads into therefrigeration or froster chamber (3), at least one of the expansionnozzle (20) adjoining the end of the supply line in the refrigeration orfroster chamber (3), wherein the metering valve (30) regulates theflow-through in the supply line, wherein the expansion nozzle (20)includes the metering valve (30) as an integrated component; wherein agas separator (70), which separates the gas components contained in theliquid carbon dioxide, is interposed in the supply line (4) for theliquid carbon dioxide.
 19. The apparatus according to claim 18,characterized in that the gas separator (70) comprises a pre-separationchamber (71) having an intake connection (73) for the two-phase carbondioxide, the connection being disposed laterally in the upper or centerregion, and an outlet connection (74) for the degassed, liquid carbondioxide, the connection being disposed in the lower region, with the gasand liquid phases being separated from one another in the separator andthe separated, gaseous carbon dioxide collecting in the upper region(75) and being carried off by way of a control head (72) disposed on thetop side and having a regulated outlet valve.
 20. The apparatusaccording to claim 19, characterized in that the intake connection (73)for generating a circulation flow in the pre-separation chamber (71) isdisposed at the cylindrical pre-separation chamber (71), diagonallyupward and tangentially to the longitudinal axis of the chamber.
 21. Theapparatus according to claim 19, characterized in that thepre-separation chamber (71) is partitioned into at least two partialchambers (75, 76) by at least one partition wall (77) disposed below theintake connection (73) and provided with apertures, and extendingtransversely to the longitudinal direction of the pre-separationchamber.
 22. The apparatus according to claim 19, characterized in thatthe control head (72) comprises a float chamber with an outlet valvethat is open to the top, whose closing element is coupled to a floatlocated in the float chamber, and in an upper position of the float inthe float chamber, the outlet valve is closed, whereas it is open in alower position of the float.
 23. The apparatus according to claim 18,characterized in that gaseous carbon dioxide separated in the gasseparator (70) is conducted into the refrigeration or froster chamber(3) by way of a separate gas line (78).
 24. The apparatus according toclaim 23, characterized in that a blocking element (79) is disposed inthe gas line (78), in front of the refrigeration or froster chamber (3).25. The apparatus according to claim 23, characterized in that theliquid and gaseous carbon dioxide exiting the gas separator (70) areconducted to the refrigeration or froster chamber (3) in a doublepipeline formed from two coaxially-nested pipes, with the gaseous carbondioxide being conducted through the outside, annular pipe.
 26. Theapparatus according to claim 18, characterized in that a blockingelement is disposed in the supply line for the liquid carbon dioxide,behind the gas separator and in front of the refrigeration or frosterchamber (3).
 27. The apparatus according to claim 18, characterized inthat a plurality of expansion nozzles (20) that are connected to thesupply line (4) for the liquid carbon dioxide are disposed in therefrigeration or froster chamber (3), above a transport device (5),particularly a conveyor belt (5), for the refrigerated or frozen items(2).
 28. The apparatus according to claim 27, characterized in that atleast two expansion nozzles (20) are disposed one behind the other inthe transport direction (R), above the transport device (5), and atleast one of the expansion nozzles (20) is directed at the conveyor belt(5) diagonally, at a downward 45° angle in the transport direction (R),and the other expansion nozzle (20) is directed diagonally downward atthe transport device (5), counter to the transport direction (R).
 29. Anexpansion nozzle (20) for making carbon-dioxide snow from liquid carbondioxide for spraying refrigerated or frozen items (2), wherein thenozzle has a discharge direction and comprises:an integrated meteringvalve (30) including a discharge aperture (32), an intake-side valveseat (33) and a valve needle (31) adjustable relative to the valve seat(33), the needle being adapted to close the metering valve (30) in thedischarge direction, the needle projecting into the discharge aperture(32) of the metering valve (30) in a closed state of the metering valve;wherein the valve seat (33) includes a discharge funnel (34) thatnarrows conically in the discharge direction; wherein the dischargefunnel (34) of the valve seat (33) includes an opening angle betweenabout 56° and about 76°, preferably 66°; wherein the valve needle (31)converges to a point at an angle between about 16° and about 30°; andwherein the valve needle (31) is guidable out of the valve base (40) ina guide channel (44) that extends from the valve chamber (41) oppositethe valve seat (33).
 30. The expansion nozzle according to claim 29,wherein the opening angle is 66°.
 31. An expansion nozzle (20) formaking carbon-dioxide snow from liquid carbon dioxide for sprayingrefrigerated or frozen items (2), wherein the nozzle has a dischargedirection and comprises:an integrated metering valve (30) including adischarge aperture (32), an intake-side valve seat (33) and a valveneedle (31) adjustable relative to the valve seat (33), the needle beingadapted to close the metering valve (30) in the discharge direction, theneedle projecting into the discharge aperture (32) of the metering valve(30) in a closed state of the metering valve; wherein the valve seat(33) includes a discharge channel (35) that narrows conically in thedischarge direction; a jet shaper (60) adjoining the discharge channel(35), in which the liquid carbon dioxide expands in a jet shape, the jetshaper including a nozzle segment having a cross section widened at anend thereof; the jet shaper (60) including on a valve side thereof ashort, cylindrical channel (61) acting as a nozzle throat (61), andadjoining thereto an expansion region (62) including a diverging crosssection having dimensions being a function of the desired jet shape; andwherein the nozzle throat (61) of the jet shaper (60) has a samediameter as the discharge channel (35) of the valve seat (33), andseamlessly adjoins the discharge channel in an the assembled state ofthe nozzle, and wherein a total length of the nozzle throat (61) and thedischarge channel (35) approximately corresponds to half of a tipdiameter of the needle tip divided by the tangent of half of an angle ofthe needle tip.
 32. An expansion nozzle (20) for making carbon-dioxidesnow from liquid carbon dioxide for spraying refrigerated or frozenitems (2), wherein the nozzle has a discharge direction and comprises:anintegrated metering valve (30) including a discharge aperture (32), anintake-side valve seat (33) and a valve needle (31) adjustable relativeto the valve seat (33), the needle being adapted to close the meteringvalve (30) in the discharge direction, the needle projecting into thedischarge aperture (32) of the metering valve (30) in a closed state ofthe metering valve; wherein the valve seat (33) includes a dischargechannel (35) that narrows conically in the discharge direction; a jetshaper (60) adjoining the discharge channel (35), in which the liquidcarbon dioxide expands in a jet shape, the jet shaper including a nozzlesegment having a cross section widened at an end thereof; the jet shaper(60) including on a valve side thereof a short, cylindrical channel (61)acting as a nozzle throat (61), and adjoining thereto an expansionregion (62) including a diverging cross section having dimensions beinga function of the desired jet shape; and wherein a nozzlethroat/discharge channel length can be changed in association with aposition of the valve needle (31).
 33. An expansion nozzle (20) formaking carbon-dioxide snow from liquid carbon dioxide for sprayingrefrigerated or frozen items (2), wherein the nozzle has a dischargedirection and comprises:an integrated metering valve (30) including adischarge aperture (32), an intake-side valve seat (33) and a valveneedle (31) adjustable relative to the valve seat (33), the needle beingadapted to close the metering valve (30) in the discharge direction, theneedle projecting into the discharge aperture (32) of the metering valve(30) in a closed state of the metering valve; wherein the valve seat(33) includes a discharge channel (35) that narrows conically in thedischarge direction; a jet shaper (60) adjoining the discharge channel(35), in which the liquid carbon dioxide expands in a jet shape, the jetshaper including a nozzle segment having a cross section widened at anend thereof; the jet shaper (60) including on a valve side thereof ashort, cylindrical channel (61) acting as a nozzle throat (61), andadjoining thereto an expansion region (62) including a diverging crosssection having dimensions being a function of the desired jet shape; andwherein the diameter of the nozzle throat (61) is selected as a functionof a nominal width of the metering valve (30) and desired operatingparameters of a refrigeration or froster chamber (3), a length of thenozzle throat (61) is selected as a function of a nominal width of themetering valve (30), an angle of the valve needle (31) and the stroke ofthe actuating drive of the valve needle (31) such that the pressure ofthe carbon dioxide flowing through the nozzle throat (61) does not dropbelow the triple-point pressure of the carbon dioxide inside the nozzlethroat (61).